In recent years, mobile communication traffic has rapidly increased with the spread of smartphones and the like, and there is a pressing need for research and development of the fifth generation mobile communication system (5G), which is expected to be put into practical use in or after 2020. Improvement of the communication system capacity is one of abilities needed in 5G, and cooperative transmission from plural transmission points (TPs), for example, is a means for realizing this ability. Specifically, for example, cooperative transmission, which is realized by: highly close arrangement of TPs, such as remote radio heads (RRHs), in a high traffic area (a hot spot); and control of plural TPs from a centralized base band unit (CBBU), may be considered.
In general, improvement of the communication system capacity due to a gain of cell splitting is expected by highly close arrangement of TPs, but practically, because the probability that the propagation environment between each TP and user equipment (UE) will be a line of sight (LOS) environment is high, interference from peripheral TPs may be increased.
Accordingly, use of cooperative multi-user-multi-input-multi-output (MU-MIMO) transmission where plural TPs, which perform cooperative transmission, simultaneously transmit signals addressed to plural pieces of UE in an area (hereinafter, referred to as a “cooperative cluster”) covered by the plural TPs may be considered. In cooperative MU-MIMO transmission, transmission weights are calculated based on estimation results of MIMO wireless channels between TPs and pieces of UE in a cooperative cluster, and transmission amplitudes and phases of antennas of the respective TPs are individually controlled by use of the transmission weights. Thereby, high communication system capacity according to the arrangement density of the TPs is able to be achieved ideally in a high quality communication environment where there is no interference between the TPs and between the pieces of UE.    Patent Literature 1: Japanese Laid-open Patent Publication No. 2011-19101    Patent Literature 2: Japanese Laid-open Patent Publication No. 2017-11486    Patent Literature 3: Japanese Laid-open Patent Publication No. 2014-27368
At most, the size of a cooperative cluster is limited to a range covered by TPs connected to a single CBBU, and is thus not limitless. Therefore, the following two techniques may be considered for a hot spot of a wide area, such as, for example, an overpopulated area, to be covered area-wise.
That is, according to the first techniques, a cooperative cluster is increased in size. In this case, station installation for connection between multiple TPs distributed over a wide area and a CBBU is difficult, and the amount of processing for scheduling and calculation of transmission weights is increased as the numbers of TPs and pieces of UE in the cooperative cluster are increased. Therefore, simply increasing the number of TPs that perform cooperative transmission to increase the size of the cooperative cluster is not realistic.
Thus, according to the second technique, plural cooperative clusters are closely arranged adjacently to each other. However, in this case, although no interference occurs between TPs in each cooperative cluster, there is a problem that interference occurs between TPs of cooperative clusters that are adjacent to each other. That is, interference between TPs occurs in the vicinity of a boundary between adjacent cooperative clusters, and the communication system capacity will be limited by this interference.
It has been known that normally: when a cooperative cluster is isolatedly arranged, by increase in the number of TPs in the cooperative cluster, the communication system capacity is able to be increased; but when plural cooperative clusters are closely arranged, even if the number of TPs in each cooperative cluster is increased, the communication system capacity is not increased much.